1. Technical Field
This invention relates generally to medical equipment, and more particularly to a system and methodology for detecting air in a liquid being delivered into a patient.
2. Background Information
An infusion system for delivering a drug or other liquid into a patient often includes an infusion device that operates to deliver the liquid at an adjustable rate or dosage. Commonly housed in a small cabinet suitable for bedside use, the infusion device operates to control the flow of liquid through a flexible tubing or other infusion conduit that extends from an IV bag or other source of the liquid to an IV needle or other cannula inserted in the patient. In addition to a linear peristaltic infusion pump, the infusion device may include microprocessor control circuitry, front panel operator controls, a display, and an alarm, with those things being integrated in a very functional unit designed to improve intravenous drug administration.
In order to guard against air bubbles flowing into the patient, the infusion device may also include an air detector. Sometimes called a bubble detector, it may take the form of an ultrasonic transmitter/receiver pair and related circuitry arranged to sense air bubbles in the infusion conduit. For that purpose, the ultrasonic transmitter and receiver occupy facing positions on opposite sides of the infusion conduit so that ultrasonic energy passes through the infusion conduit in traveling from the transmitter to the receiver. Whenever an air bubble (i.e., a column of air) moves within the infusion conduit to a position between the transmitter and receiver, it causes a recognizable variation in the receiver output (i.e., bubble detector output) and if that variation is sufficient to signify an air bubble of unacceptable size, control circuitry stops the infusion pump and activates the alarm.
Unacceptable air bubble size may differ, however, and so it would be advantageous to have some way to set that value. Then an operator could select a bubble size most appropriate for a particular infusion situation and that would, among other things, avoid the nuisance alarms accompanying too sensitive a setting. But existing infusion devices often use fixed-length bubble detector schemes so that sensitivity is fixed at some predetermined value such as a three-eighths inch bubble length (i.e., about fifty microliters in some commonly used infusion conduit).
One common configuration includes a transmitter and receiver about five-eighths inch long and circuitry configured to pass one-eighth inch long bubbles while stopping three-eighth inch bubbles. The circuitry can recognize variations in bubble detector output from a predetermined threshold value signifying that air between the transmitter and receiver is interrupting 20% of the five-eighths inch bubble detector length (i.e., a one-eighth inch long air bubble), to a predetermined maximum value indicating 60% interruption (i.e., a three-eighths inch long air bubble). When a three-eighths inch long bubble is detected, the control circuitry stops the infusion pump.
So, infusion device sensitivity to air bubble size (i.e., the acceptable air bubble size) is not adjustable. In addition, minimum sensitivity (i.e., maximum acceptable air bubble size) is dependent on the length of the transmitter/receiver pair, a longer transmitter/receiver pair being required to detect longer air bubbles and establish a threshold value much greater than three-eighths inch. But changing the transmitter/receiver pair and associated hardware may be quite costly and inconvenient. So, some other way is needed to reduce infusion device sensitivity in order to pass air bubbles up to an unacceptable size that is larger than the fixed threshold value, and perhaps even larger than the limit imposed by the length of the transmitter/receiver pair.